Alkylaromatic fluids have been proposed for use as certain types of functional fluids where good thermal and oxidative stability are required. For example, U.S. Pat. No. 4,714,794 (Yoshida) describes the monoalkylated naphthalenes as having excellent thermal and oxidative stability, low vapor pressure and flash point, good fluidity and high heat transfer capacity and other properties which render them suitable for use as thermal medium oils. The use of a mixture of monoalkylated and polyalkylated naphthalenes as a base for synthetic functional fluids is described in U.S. Pat. No. 4,604,491 (Dressler) and Pellegrini U.S. Pat. Nos. 4,211,665 and 4,238,343 describe the use of alkylaromatics as transformer oils.
The alkylated naphthalenes are usually produced by the alkylation of naphthalene or a substituted naphthalene in the presence of an acidic alkylation catalyst such as a Friedel-Krafts catalyst, for example, an acidic clay as described in Yoshida U.S. Pat. No. 4,714,794 or Dressler U.S. Pat. No. 4,604,491 or a Lewis acid such as aluminum trichloride as described in Pellegrini U.S. Pat. Nos. 4,211,665 and 4,238,343. The use of a catalyst described as a collapsed silica-alumina zeolite as the catalyst for the alkylation of aromatics such as naphthalene is disclosed in Boucher U.S. Pat. No. 4,570,027. The use of various zeolites including intermediate pore size zeolites such as ZSM-5 and large pore size zeolites such as zeolite L and ZSM-4 for the alkylation of various monocyclic aromatics such as benzene is disclosed in Young U.S. Pat. No. 4,301,316 and U.S. Pat. No. 4,962,256.
In the formulation of functional fluids based on the alkyl naphthalenes, it has been found that the mono-substituted naphthalenes are often preferred because they provide the best combination of properties in the finished product: there are fewer benzylic hydrogens than the corresponding di-substituted or polysubstituted versions and as a result, the mono-alkylated derivatives have better oxidative stability. The mono-substituted naphthalenes usually have a kinematic viscosity in the range of about 5-8 cSt (at 100.degree. C.) when working with alkyl substituents of about 14 to 18 carbon atoms chain length and this means that they usually have to be blended with more viscous materials to produce the higher viscosity fluids. The mono-alkylated naphthalenes tend to be less stable than the corresponding poly-alkylated derivatives but they do, nevertheless, have certain utilities. They have higher viscosities and can therefore be used for blending with the mono-alkylated materials for obtaining higher viscosity products; in addition, the presence of more alkyl groups tends to improve their viscosity index (VI), particularly when the alkyl groups are derived from high VI materials.